The present invention relates to a technique and structure therefore, for improving the power output and stability of heterojunction bipolar transistors. One of the advantages of HBTs based on III-V materials (gallium arsenide (GaAs) and indium phosphide (InP)) is their high power density capability, which is critical in low cost and high power amplifiers for commercial and military applications. However, the inefficient heat dissipation in such devices has been the major constraint to the power density advantage. The inefficient heat dissipation is due to the fact that the thermal conductivity of GaAs is approximately one third (1/3) that of silicon. The heat in a GaAs HBT is generated in a region under the emitter mesa, and then spreads out normally through the semi-insulating GaAs substrate as shown by the arrows 100 in FIG. 1. This generation of heat is commonly referred to as self-heating, a term of art, and is due to the flow of current across the emitter/base and base/collector junctions which is of course due to voltage drops across the junctions. The flow of current across the junctions dissipates power in the form of heat. Elevated junction temperature due to the device self-heating not only degrades power and gain performance, but also the device lifetime. Therefore, it is of utmost importance to find an efficient method of removing joule heat from the device.
There are two conventional techniques to improve the dissipation of joule heat in HBTs. The first technique is a flip-chip assembly approach (FIG. 2). The second technique is commonly referred to as a thermal shunt as it uses thick-plated air bridges for spreading the heat (FIG. 3).
The flip-chip assembly approach is a technique to extract the joule heat generated within the device from above through metal contacts and to place these contacts on a large heat sink. This is most readily achieved by turning the device upside-down onto the heat sink. Accordingly, this process is well known in the art as flip-chip mounting. Flip-chip mounting of a HBT requires contacts to the heat sink to be placed directly over the emitter fingers. In conventional flip-chip mounting in GaAs or other III-V devices, the contacts provide the path to thermal ground and electrical ground. This configuration, however, does not allow for the placement of an off-finger ballast resistor between the emitter fingers and the ground. Ballast resistors are required for stable high power operation. To this end, the flip-chip mounting approach, while being quite efficient in improving thermal performance characteristics, is a challenging assembly process. The small physical feature sizes of the emitter mesa (typically on the order of 1-3 microns) make manufacturability of the flip-chip process difficult in terms of alignment of the device contacts to the heat-sink contacts, since neither contact is visible during the process. Accordingly, flip-chip mounting has clear disadvantages as a technique to remove heat in a device. For power HBT structures with multiple fingers, the requirement of a ballast resistor at each emitter finger further exacerbates the problem.
The use of a thermal shunt (shown in FIG. 3) as a technique to dissipate joule heat in a HBT structure has also proved problematic. The manufacturability of the shunt metal and the size of the metal post connecting the emitter mesa to the shunt metal limit the amount of improvement in the HBT thermal characteristics. Clearly, the thicker the shunt and the larger the post, the lower the thermal impedance, but the more difficult reproducible results are to achieve in large scale processing. Further details of both of the above-captioned prior techniques for dissipating heat in HBT structures can be found in "Very High Power Density CW Operation Of GaAs/AlGaAs Microwave HBTs", IEEE Electronic Device Letters, Vol., 14, No. 10, October 1993, pgs. 493-495; and "Bump Heat Sink Technology, A Novel Assembly Technology Suitable For Power HBTs", GaAs IC Symposium, pgs. 337-340, the disclosures of which are specifically incorporated herein by reference.
Finally, a conventional HBT structure has a symmetric layout that permits uniform power dissipation through the emitter mesa, the base and the collector, as shown in FIG. 2. This symmetry is relatively standard as can been seen in FIGS. 2, and 3 as well, where a line of symmetry 101, 201, 301 can be drawn through the emitter mesa and a mirror image is found on either side of the emitter mesa. In this configuration, the heat is concentrated in a region under the emitter mesa. The narrow strip of metal overlay (shown in FIG. 4 at 400) does not suffice as a good thermal path in this design. The thermal dissipation characteristics of the device cannot be improved by enlarging the emitter metallization or the metal overlay thereof, because the design is symmetrical. Therefore, a structure is needed which will improve the dissipation of heat from the device.
Accordingly, what is needed is a technique for improving the thermal dissipation capabilities in HBT structures for power applications which overcomes the difficulties in processing and reproducibility of the prior art techniques.